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How High Is Too High to Drive? Law Enforcement Agencies Tackle Complex Issue.

A drug test that proves someone smoked marijuana three days ago doesn't prove they are impaired now.

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One of the issues to emerge with the widespread legalization of marijuana is in the area of law enforcement and safety.

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Specifically, how to tell if a driver is impaired by marijuana use.

The easy solution is simply not to drive when partaking of cannabis. But given that the same message has not worked so well with alcohol, law enforcement agencies across the country are understandably concerned about how to address the problem.

Obviously, not long ago when marijuana was illegal everywhere it was a simple matter of testing for any presence of THC, the active chemical in marijuana that causes the high. Now, more refinement is needed.

But the issue is complicated. Not all scientists agree on what constitutes being impaired. Even the federal government is having trouble tackling the issue. And state laws are all over the map.

Related: The Feds and the NFL Both Forbid Marijuana. Both Are Studying It as an Alternative to Opiods.

Congressional report.

In a report to Congress this summer, the National Highway Traffic Safety Administration (NHTSA) made a series of recommendations. They fell into three main categories.

1. Training officers on detecting and measuring the impairment of a driver using field sobriety tests and technology. The agency has developed training to help officers in detecting possible impairment of drivers by marijuana, called “Drugs That Impair Driving”

2. Continued research to develop a standard for drug impairment while driving, similar to the levels of alcohol present in the blood stream

3. Encourage states where marijuana is legal to continue efforts to collect data on the number of marijuana-related driving impairment arrests

The federal agency also encouraged states to separate drug-related driving arrests from those involving alcohol. Further, they suggest clearly defining which drug was involved in the incident.

In short, the report calls for more study. It also warned that state laws on impairment from marijuana appear “to have been based on something other than scientific evidence.”

Related: To Understand the Forces Favoring and Opposing Legal Marijuana, Study Arizona.


Various studies have reached different conclusions about marijuana’s impact on driving.

Part of the issue is how marijuana works. The “high” effects can vary depending on whether it is smoked or ingested. In some cases, the effects can wear off in as little as 30 minutes, even if traces of THC are still in the blood or saliva.

And studies have been all over the map. A University of Colorado study in 2014 found that the number of fatal accidents involving marijuana-impaired drivers had jumped almost 5 percent between 2009 and 2011.

But a study from Virginia conducted by the NHTSA in 2015 found that drivers under the influence of marijuana had the same chance of having an accident as those who were completely sober.

And there’s the issue of combination. According to the Denver Post, 36 percent of drivers involved in fatal accidents who tested positive for marijuana had also tested positive for alcohol.

Related: Oakland Explores Reparations for Those Persecuted by the War on Drugs

All this hasn’t stopped states from trying. Colorado and Washington, both states where adult-use marijuana is legal, have set a limit of 5 nanograms per milliliter as the level of impairment. But even there, differences occur.

In Colorado, a law enforcement officer can use that level to charge someone with impairment, but it’s up to a judge or jury to decide if the driver was truly impaired based on other evidence. In Washington, it’s more like the limit set on alcohol, and no further proof of impairment is needed.

John Jackson, police chief of Greenwood Village in Colorado, told the Post that debate continues on what constitutes actual impairment. “We’re in the infancy with this, and it’s very much an unknown since we don’t have the data,” he said.

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